Includes Limonius spp., Agriotes spp., and other wireworm species
Pest description and crop damage Wireworms (Order Coleoptera: Family Elateridae) are the most important soil-dwelling pests infesting crops in the PNW. The adults, known as click beetles, do little or no damage; they feed on flowers. The larval or immature stages cause major damage to seedlings and the underground portions of many annual crops, including hemp. The larvae are shiny white at first, but later become straw color or light brown. They look wiry and are about 1 inch long when mature depending on species.
Biology and life history Depending on species, wireworms may require two to six years to mature. They overwinter 12 to 24 inches deep in the soil and return near the surface in spring to resume feeding. Mature larvae pupate in the soil, developing into adults that will remain in the soil until the following spring, when they emerge, mate, and lay eggs. Because the female beetles fly very little, infestations do not spread rapidly from field to field. Soil temperature is important to wireworm development and control. Larvae start to move upward in the spring, when soil temperature at the 6 inch depth reaches 50°F. Later in the season, when temperatures reach 80°F and above, the larvae tend to move deeper than 6 inches, where most remain until the following spring. For more information, see https://catalog.extension.oregonstate.edu/em9166. In hemp, they can cause wilting in small plants, especially on fields that follow pastures.
Scouting and thresholds Ideally, the presence of wireworm in a field should be determined before using control measures. However, effectively determining wireworm density is difficult and/or impractical on the large fields that are the rule in many areas. Crop sequence also is important; thus, planting a susceptible crop such as hemp immediately following pasture, grass hay, red clover, or grain is risky. In fields that are plowed deeply in the fall, wireworms will turn up during plowing. They may be detected by following behind the plow and checking for them in the turned up soil. Fall plowing, however, is becoming much less common. There are no established treatment thresholds for wireworms in hemp.
Management-cultural and biological controls
Crop rotation is an important tool for wireworm control. Wireworms tend to increase rapidly among red and sweet clover and small grains (particularly barley and wheat). Birds feeding in recently plowed fields destroy many wireworms. However, in seriously infested fields this does not reduce the overall pest population. There are no parasites or biological insecticides known to be effective in wireworm control.
For more information, see http://cdn.intechopen.com/pdfs/28267.pdf